This to the general process and means thereof for applying and trimming maskant with the aid of one or more lasers on structural forms or workpieces for the purpose of performing chemical milling on them. One of the purposes of performing chemical milling, but not limited to this purpose, is to produce an integral ribbed stiffened structure such as an isogrid or waffle structure in which bosses and other local features are optionally incorporated into the rib stiffening pattern.
Chemical milling normally comprises the steps of:
(a) preparing the surfaces of the workpiece to be chemically milled by dust or sand blasting the surface and then wiping it with polypropyl alcohol or an equivalent solvent to respectively roughen and clean the surface to thereby promote maskant adhesion,
(b) spraying the entire surface of the workpiece with one or more coats of maskant including an optional topcoat, one or both of which are impervious to the etchant bath in which the workpiece is to be immersed,
(c) scribing the maskant as with, fixtured and guided, manually operated Exacto knives or a numerically controlled (NC) laser cutter,
(d) stripping away the maskant from areas that are to be chemically milled,
(e) performing the chemical milling by immersing the workpiece in an etchant vat and creating relative motion between the etchant and the workpiece as by rotationally actuating the workpiece and/or agitating the etchant,
(f) removing the maskant when the chemical milling is completed,
(g) repeating steps (a) to (f) above for subsequent chemical milling cuts on either all or part of the area previously chemically milled or on areas that were not previously chemically milled.
U.S. Pat. No. 2,739,047 issued Mar. 20, 1956 to M. C. Sans discloses a process and a resultant article in which my invention applies and improves both the process and the resultant article. The processes and products in the Sans patent are representative of, but not limited to, the processes and products to which my invention applies.
An understanding of the background of my invention is further provided by reference to processes and products produced by deep cut chemical milling. Each deep chemical milling cut requires the application of 2 to 3 coats of maskant, as with a hot airless spray gun, followed by curing, drying, scribing and finally peeling away of maskant from those areas to be exposed to the etchant. When a cut is completed, the remaining maskant is stripped away and the part is cleaned and typically dust blasted. This process must be repeated for each successive chemical milling cut. After the first cut, maskant scribing or laser cutting may have to take place on non-flat or irregular surfaces.
In place of applying maskant over an entire part, this invention applies maskant accurately and directly where needed thereby also eliminating the need for scribing or laser cutting and peeling.
This invention includes the use of a robot or a multi axis milling machine type actuator system to transport a maskant applicator and laser head relative to a part on which the maskant is to be applied.
A laser assisted applicator head consists of a maskant spraying, painting or application device plus one or more scanned laser beams. The maskant applicator device can be an airless brush, a hot airless spray nozzle, an air driven spray gun, a roller, an air brush or any applicator or dispenser of maskant. The scanned laser beam or beams, on one or more sides of the maskant applicator device, are fan beam scanned in the direction of the laser and maskant application head movement and are positioned to act on the maskant spray at the instant the spray is airborne and/or impinging on the workpiece surface. The effect of the scanned laser beam is in part to disintegrate the excess maskant that would otherwise adhere to the surface and cause an irregular edge along the strip of maskant applied by the translation of the maskant application device. In addition the scanned laser beam trims and shapes the edges of the maskant at the very instant maskant is applied to the surface and thereby further contributes to the positional and width accuracy of the maskant strip. The same procedure is used for successive maskant coats.
Advantages derived from the use of this invention include, but are not limited to:
1. Quality of resultant products are improved by the high accuracy with which the maskant can be laid and positioned. PA0 2. Material savings, especially in the maskant; approximately 90% of the maskant is saved. PA0 3. Speed of the maskant application is much higher than possible by manual means. PA0 4. Quality is improved by virtue of eliminating all manual operations. PA0 5. Due to uniform feeds and speeds made possible by machine control the maskant thickness can be more accurately controlled than possible by conventional manually controlled application means. (i.e. The estimated maskant thickness may be reduced to 0.005 to 0.008 inch from 0.005 to 0.015 inch.) PA0 6. The second maskant coat, such as the topcoat, can be extended to cover the longitudinal edges of the strip which are ordinarily exposed to the maskant (i.e. with no topcoat) in conventional scribing or laser cutting were these operations are performed after all coating applications are first completed. PA0 7. Ventilation of spray fumes and laser created debris is minimized by virtue of the generation of a large percent of the fumes and debris off the application surface where they are already airborne and easily entrained by ventilating air flows. PA0 8. In place of two or more trimming laser scanners, one laser scanner can be used on one side of the maskant application nozzle to affect trimming on that side of the maskant strip which is made substantially narrower than the space between the impingement points on the application surface of the laser beams. A second parallel maskant strip application is overlapped on this strip and the opposite edge is then trimmed. PA0 9. Risks are eliminated for maskant lifts due to operator errors while stripping the maskant.